1. Field of the Invention
This invention relates to a novel method of preparing indicator dyes and to novel intermediates useful in the preparation thereof.
2. Description of the Prior Art
Dyes which undergo a change in spectral absorption characteristics in response to a change in pH are well known in the art, and frequently, are referred to as indicator or pH-sensitive dyes. Typically, these dyes change from one color to another, from colored to colorless or from colorless to colored on the passage from acidity to alkalinity or the reverse and are commonly employed in analytical chemical procedures to measure changes in pH value. Among the indicator dyes most widely used is the group derived from phthaleins as exemplified by phenolphthalein, thymolphthalein, cresolphthalein and 1-naphtholphthalein.
Though various methods have been used for synthesizing phthalein indicator dyes, they are generally prepared via the Friedel-Crafts reaction by condensing the selected phenol or 1-naphthol with phthalic or naphthalic acid, their anhydrides or acid chlorides at elevated temperature in the presence of a suitable catalyst, for example, zinc chloride or sulfuric acid to yield the corresponding symmetrical, i.e., bis-phenol or bis-naphthol phthalide or naphthalide. Unsymmetrical phthalieins and mixed phthalein indicator dyes, such as, benzene-resorcinol phthalide are prepared in a similar manner by condensing an o-aroyl benzoic acid dye intermediate with the selected phenol also in the presence of a suitable catalyst, such as, sulfuric acid at elevated temperature.
The present invention is concerned with a new method of synthesizing certain phthalein indicator dyes, namely, 1-naphthol phthalides and naphthalides.